The present invention concerns a composite sheet material. The object of the present invention is to provide a sheet material which is superior in terms of dimensional stability and shape retention, and which also has stable and superior bonding characteristics.
The present invention provides a synthetic resin sheet which can be bonded to various types of metal sheets or foils, or which can be used for other purposes.
Polytetrafluoroethylene resins have superior heat resistance, chemical resistance and electrical characteristics, and are widely used in a variety of fields. One such polytetrafluoroethylene resin product is porous, expanded polytetrafluoroethylene disclosed in U.S. Pat. No. 3,953,566, incorporated by reference. This substance not only has the abovementioned characteristics of polytetrafluoroethylene, but is also endowed with air permeability, softness, biocompatibility and good bonding characteristics. Accordingly, this material is widely used in waterproof, water-vapor-permeable clothing, artificial blood vessels, various types of filters, and various types of industrial packing.
Specifically, the aforementioned porous, expanded polytetrafluoroethylene has fine continuous pores. Bonding of this material is achieved by an anchoring effect resulting from penetration and fixing within the fine porous structure of the resin. However, since expanded polytetrafluoroethylene intrinsically has an extremely low surface energy, it is difficult to wet the polytetrafluoroethylene with various types of adhesives. As a result, the bonding strength is generally low. Accordingly, in cases where a high bonding strength is required, it is usually necessary to first etch the surface of the expanded polytetrafluoroethylene with an alkali metal or other reactant.
Because the expanded polytetrafluoroethylene exhibits considerable cold flow and thermal expansion, it is generally difficult to use expanded polytetrafluoroethylene in cases where a high degree of dimensional stability is required, and these properties are especially conspicuous in the case of highly porous polytetrafluoroethylene, because this material has numerous pores in its structure. Accordingly, in cases where such highly porous polytetrafluoroethylene is to be utilized in a bonding sheet, a method is generally used in which the porous polytetrafluoroethylene is impregnated with another resin which has a high surface energy.
However, even in the case of materials formed by impregnating porous, expanded polytetrafluoroethylene with another resin as described above, it is extremely difficult to insure that the impregnating resin fills the entire interior of the porous structure of the polytetrafluoroethylene. Because of the poor wetting properties of polytetrafluoroethylene, polytetrafluoroethylene may be exposed at the surface of a composite material thus formed by impregnating porous polytetrafluoroethylene with another resin. If a sheet with polytetrafluoroethylene thus exposed at the surface is used for bonding purposes, the distribution of the adhesive will be nonuniform as a result of the fluid behavior of the impregnating resin. This can lead to unsatisfactory bonding and insufficient strength. For example, in a case where two sheets of sheet iron are thermally bonded to both sides of the aforementioned impregnated polytetrafluoroethylene bonding sheet, the impregnating resin, which has fluidity as a result of softening due to heating, migrates to the iron sheets, which have a high surface energy, so that very little impregnating resin remains on the polytetrafluoroethylene.
The abovementioned phenomenon is especially conspicuous in cases where two or more impregnated polytetrafluoroethylene bonding sheets of the type described above are used stacked together. In such cases, almost no impregnating bonding resin remains at the interfaces between the stacked impregnated polytetrafluoroethylene sheets. As a result, there may be little or no adhesive strength.
Accordingly, although the aforementioned special properties of porous, expanded polytetrafluoroethylene are fully understood, such porous polytetrafluoroethylene suffers from a drawback in that it generally cannot be used as a bonding structural material of the type described above.